The knot is the unit of measurement for boat speed. One knot corresponds to 1 mile per hour (1.852 km/h). This unit is used in both marine and aerial applications. But why the term knot?
In the early days of marine technology, long before we equipped our boats with electronic sensors, the solution to measuring speed was to throw a board connected by a rope to the back of the boat. The board, weighted on one side, stood vertically in the water. The rope was marked with knots, evenly spaced.
The board is thrown into the water and the time of an hourglass, we let the string spin. Then when the hourglass is empty. The string is pulled up and the number of knots is counted.
This number of knots gives the instantaneous speed of the boat.
The expression has remained.
Video explanation on board the Hermione where a knot loch was rebuilt on the occasion of its return to Brest.
